1. Field of the Invention
This invention relates generally to a method and apparatus for refrigeration system control and, more particularly, to a method and apparatus for refrigeration system control utilizing electronic evaporator pressure regulators and a floating suction pressure set point at a compressor rack.
2. Discussion of the Related Art
A conventional refrigeration system includes a compressor that compresses refrigerant vapor. The refrigerant vapor from the compressor is directed into a condenser coil where the vapor is liquefied at high pressure. The high pressure liquid refrigerant is then generally delivered to a receiver tank. The high pressure liquid refrigerant from the receiver tank flows from the receiver tank to an evaporator coil after it is expanded by an expansion valve to a low pressure two-phase refrigerant. As the low pressure two-phase refrigerant flows through the evaporator coil, the refrigerant absorbs heat from the refrigeration case and boils off to a single phase low pressure vapor that finally returns to the compressor where the closed loop refrigeration process repeats itself.
In some systems, the refrigeration system will include multiple compressors connected to multiple circuits where a circuit is defined as a physically plumbed series of cases operating at the same pressure/temperature. For example, in a grocery store, one set of cases within a circuit may be used for frozen food, another set used for meats, while another set is used for dairy. Each circuit having a group of cases will thus operate at different temperatures. These differences in temperature are generally achieved by using mechanical evaporator pressure regulators (EPR) or valves located in series with each circuit. Each mechanical evaporator pressure regulator regulates the pressure for all the cases connected within a given circuit. The pressure at which the evaporator pressure regulator controls the circuit is adjusted once during the system start-up using a mechanical pilot screw adjustment present in the valve. The pressure regulation point is selected based on case temperature requirements and pressure drop between the cases and the rack suction pressure.
The multiple compressors are also piped together using suction and discharge gas headers to form a compressor rack consisting of the multiple compressors in parallel. The suction pressure for the compressor rack is controlled by modulating each of the compressors on and off in a controlled fashion. The suction pressure set point for the rack is generally set to a value that can meet the lowest evaporator circuit requirement. In other words, the circuit that operates at the lowest temperature generally controls the suction pressure set point which is fixed to support this circuit.
There are, however, various disadvantages of running and controlling a system in this manner. For example, one disadvantage is that the requirement for the case temperature generally changes throughout the year. This requires a refrigeration mechanic to perform an in-situ change of evaporator pressure settings, via the pilot screw adjustment of each evaporator pressure regulator, thereby further requiring re-adjustment of the fixed suction pressure set point at the rack of compressors. Another disadvantage of this type of control system is that case loads change from winter to summer. Thus, in the winter, there is a lower case load which requires a higher suction pressure set point and in the summer there is a higher load requiring a lower suction pressure set point. However, in the real world, such adjustments are seldom done since they also require manual adjustment by way of a refrigeration mechanic.
What is needed then is a method and apparatus for refrigeration system control which utilizes electronic evaporator pressure regulators and a floating suction pressure set point for the rack of compressors which does not suffer from the above mentioned disadvantages. This, in turn, will provide adaptive adjustment of the evaporator pressure for each circuit, adaptive adjustment of the rack suction pressure, enable changing evaporator pressure requirements remotely, enable adaptive changes in pressure settings for each circuit throughout its operation so that the rack suction pressure is operated at its highest possible value, enable floating circuit temperature based on a product simulator probe, and enable the use of case temperature information to control the evaporator pressure for the whole circuit and the suction pressure at the compressor rack. It is, therefore, an object of the present invention to provide such a method and apparatus for refrigeration system control using electronic evaporator pressure regulators and a floating suction pressure set point.
In accordance with the teachings of the present invention, a method and apparatus for refrigeration system control utilizing electronic evaporator pressure regulators and a floating suction pressure set point is disclosed. To achieve the above objects of the present invention, the present method and apparatus employs electronic stepper regulators (ESR) instead of mechanical evaporator pressure regulators. The method and apparatus may also utilize temperature display modules at each case that can be configured to collect case temperature, product temperature and other temperatures. The display modules are daisy-chained together to form a communication network with a master controller that controls the electric stepper regulators and the suction pressure set point. The communication network utilized can either be a RS-485 or other protocol, such as LonWorks from Echelon.
In this regard, the data is transferred to the master controller where the data is logged, analyzed and control decisions for the ESR valve position and suction pressure set points are made. The master controller collects the case temperature for all the cases in a given circuit, takes average/min/max (based on user configuration) and applies PI/PID/Fuzzy Logic algorithms to decide the ESR valve position for each circuit. Alternatively, the master controller may collect liquid sub-cooling or relative humidity information to control the ESR valve position for each circuit. The master controller also controls the suction pressure set point for the rack which is adaptively changed, such that the set point is adjusted in such a way that at least one ESR valve is always kept substantially 100% open.
In one preferred embodiment, an apparatus for refrigeration system control includes a plurality of circuits with each of the circuits having at least one refrigeration case. An electronic evaporator pressure regulator is in communication with each circuit with each electronic evaporator pressure regulator operable to control the temperature of each circuit. A sensor is in communication with each circuit and is operable to measure a parameter from each circuit. A plurality of compressors is also provided with each compressor forming a part of a compressor rack. A controller controls each evaporator pressure regulator and a suction pressure of the compressor rack based upon the measured parameters from each of the circuits.
In another preferred embodiment, a method for refrigeration system control is set forth. This method includes measuring a first parameter from a first circuit where the first circuit includes at least one refrigeration case, measuring a second parameter from a second circuit where the second circuit includes at least one refrigeration case, determining a first valve position for a first electronic evaporator pressure regulator associated with the first circuit based upon the first parameter, determining a second valve position for a second electronic evaporator pressure regulator associated with the second circuit based upon the second parameter, electronically controlling the first and the second evaporator pressure regulators to control the temperature in the first circuit and the second circuit.
In another preferred embodiment, a method for refrigeration system control is set forth. This method includes a lead circuit having a lowest temperature set point from a plurality of circuits where each circuit has at least one refrigeration case, initializing a suction pressure set point for a compressor rack having at least one compressor based upon the identified lead circuit, determining a change in suction pressure set point based upon measured parameters from the lead circuit and updating the suction pressure based upon the change in suction pressure set point.
In yet another preferred embodiment, a method for refrigeration system control is also set forth. This method includes setting a maximum allowable product temperature for a circuit having at least one refrigeration case, determining a product simulated temperature for the circuit, calculating the difference between the product simulated temperature and the maximum allowable product temperature, and adjusting the temperature set point of the circuit based upon the calculated difference.
Use of the present invention provides a method and apparatus for refrigeration system control. As a result, the aforementioned disadvantages associated with the currently available refrigeration control systems have been substantially reduced or eliminated.